The present invention is related to an improved high power ultraviolet light source and a method of manufacturing the light source. More particularly, the present invention is related to a Group III nitride based light emitting diode which has improved efficiency and decreased joule heating.
Due to their wide and direct band gaps, group-III nitrides are promising semiconductor materials for a wide range of technological applications in optoelectronics, as well as high power-, high frequency- and high temperature electronic devices.
Recently, deep ultra-violet light emitting diodes, with peak emission wavelengths of 200-365 nm, have become a source of extensive research and development and significant commercialization efforts are underway. These deep UV LED's are being considered in such diverse areas as bio-medical research, water purification, air purification, bio-detection, solid state lighting and many others as described in Gil (Ed.) Group III Nitride Semiconductor Compounds, Physics and Applications; Series on Semiconductor Science and Technology, vol. 6, Oxford Science Publications, Oxford, 1998. Full commercial exploitation of deep UV LED's has been thwarted by extreme device self heating or joule heating. The propensity for joule heating in such devices stems from the fact that the epilayers needed for proper wavelength of emission requires very high aluminum content. With increasing aluminum content the doping efficiency decreases significantly which increases the resistance of the epilayer. The highly resistive epilayer causes a phenomenon commonly known as current crowding wherein the current injection occurs primarily near the n-contact perimeter close to the mesa side walls as detailed in Jpn. J. Appl. Phys. 41 (2002) pp. 5083-5087.
By way of example, group III-nitride based visible LED's and group III-nitride based UV LED's can be compared. The sheet resistance of n-GaN is 10 ohms/sq compared to 250 ohms/sq for 60% n-AlGaN. Thus, in deep UV LEDs with n-AlGaN the device differential resistance is very high which causes the optical power to have premature saturation and heating. In addition to the optical inefficiencies the heating degrades the device reliability and shortens product lifetimes.
There has been an ongoing desire for a deep UV LED which does not suffer illumination inefficiencies or heat related issues.